Submitted to: International Association on Water Quality
Publication Type: Proceedings
Publication Acceptance Date: September 4, 1998
Publication Date: N/A
Agriculture is the major contributor of nitrogen to the Mississippi River and hypoxia in the Gulf of Mexico. Knowing the distribution of nitrogen sources and losses is critical to identifying potential solutions. This paper maps the major agricultural sources and losses of nitrogen in the Basin including inorganic fertilizer, manure, and atmospheric deposition, and in situ sources of mineralizable nitrogen from soil organic matter and nitrogen fixed by legumes. Distribution of nitrogen losses including crop harvests, losses to the atmosphere through volatilization of manure and inorganic fertilizer, and plant senescence are shown. National data bases used include the State Soils Geographic Database, 1992 Census of Agriculture, and the National Atmospheric Deposition Program/National Trends Network. Hydrologic units with the largest residual contributions are in the Upper Mississippi and parts of the Missouri Regions. Nitrogen fertilizer, mineralizable nitrogen in soils, and legume fixation constitute the major sources in these regions. When residual contributions are compared to inputs, the northern hydrologic regions utilize a greater fraction of the inputs to produce crops than do the southern hydrologic regions. Consequently, further reductions of inorganic fertilizer in the Upper Mississippi Region may not provide substantial improvements in residual nitrogen. Residual contributions to the Arkansas/Red and Tennessee River Regions appear to be greatest where manure inputs are largest. Solutions to excess nitrogen, particularly from manure sources, in the southern hydrologic regions may provide good opportunities for reduction in nitrogen loads to the Basin.